Conventionally, in a retractable-lens camera which is provided with a retractable lens, the zooming and focusing of the lens is typically carried out upon extension of the lens by a lens-extension mechanism. For a lens-extension mechanism for a camera of this type, an arrangement in which an annular drive ring with a cam is rotated, and a member that retains the lens is provided with a translatory movement mechanism that moves the lens along the optical axis while inhibiting rotation of the same is widely used. However, with miniaturization of the image pickup means such as a CCD and an increase in the number of pixels, this arrangement for moving the annular drive ring is now required to have high accuracy in the positions of the focus lens and the zoom lens along the optical axis.
On the other hand, when the lens is extended from its retracted state, the lens barrel is likely to be touched on the outer surface by the user, and therefore a lens barrel is required which has a strong and robust structure so that it cannot be damaged or broken nor dislocated in position even when touched. However, the stronger the lens barrel is structured, the greater load the actuator for extending the lens undergoes. In view of this, the actuator is implemented by a DC motor or a stepping motor and is decelerated at a reduction ratio of hundreds using a number of gears to drive the lens with increased torque.
However, when a force is applied to component parts of the camera, there occur backlash or play of the gears and the drive ring and play of the cam pin in the cam groove, causing slight deformations of these members or parts. Consequently, when the lens is driven, a difference (position difference) occurs in the position of the lens between when the lens is driven to move in one direction and when it is driven to move in the opposite direction. Specifically, such position difference includes relatively simple hysteresis in the lens position along the optical axis, and a shift or inclination of the lens in a direction perpendicular to the optical axis and in the rotational direction.
Since the lens cannot be accurately controlled to proper positions without such hysteresis taken into account, a sensor capable of detecting the absolute position of the lens is installed, or to stop the lens, control is provided so that the lens is always moved in a predetermined direction before it is stopped. When the lens is moved in the opposite direction to that direction, a hysteresis eliminating operation is carried out such that the lens is moved a little beyond an intended stop position and then reversed and returned by the same distance to the stop position. Moreover, when the lens undergoes a position difference such as shift or inclination, an image formed by the lens changes so that an image shake occurs when the driving direction of the lens is reversed.
As one of techniques of performing focus adjustment using an image pickup signal in an image pickup apparatus, there has been proposed a technique in which, when auto focus is carried out, an image before a signal is captured from an image pickup device is stored for focus adjustment, and the stored image is displayed on a display screen of an image display means when focus adjustment is carried out (see Japanese Patent Publication No. 3302132, for example). A method of displaying an image output from an image pickup device on an image display means is referred to as “through display”. An example of the through display is that in the case of a moving object, a displayed image is changed according to the movement of the object. On the other hand, a method of temporarily restraining the through display and then displaying the stored image on the screen is referred to as “freeze display”. For example, according to the freeze display, the stored image is displayed in a manner not responding to movement of the object even when the object is moving.
Further, as auto focus (AF) according to the so-called mountain-climbing method, a technique has been disclosed in which, when a focus evaluation value as data on high frequency components of frequencies of an image pickup signal output from an image pickup device has decreased, the through display is switched to the freeze display, whereas, when the focus evaluation value has increased, the through display is carried out (see Japanese Laid-Open Patent Publication (Kokai) No. 2003-32521, for example). This technique causes only an image in focus for which a high focus evaluation value is given to be displayed, while inhibiting an image out of focus from being displayed.
However, there have been the following problems with the above-described examples of the prior art:
In the case where the freeze display is carried out all the time during focus adjustment, for example, in shooting a moving object, it is difficult to follow up the moving object while viewing the display screen. Further, in the case where the freeze display is carried out when the focus evaluation value has decreased, if the object obliquely moves away from or toward the camera, it is also difficult to follow up the object on the display screen, and thus, when shooting a moving object, it is desirable to avoid the freeze display from being carried out, to the maximum possible extent.
In the case of the mountain-climbing method in particular, to detect the direction in which the image comes into focus, it is necessary to violently or quickly move the focus lens forward and backward, such as wobbling. However, in the case where the through display is carried out only when the focus evaluation value has increased, if there occurs a position difference of the lens such as a shift or inclination, there is the possibility that the through display is carried out irrespective of the direction in which the lens is moving, so that the displayed image shakes when the focus evaluation value is just below or above its top value, giving an unpleasant feeling to the user.
In view of the above described problems with the prior art, it is an object of the present invention to provide an image pickup apparatus and a control method which enable a user to follow up an object while viewing a display screen, and can prevent an image shake from being reflected on the display screen, and a control program for implementing the control method.